1. Field of the Invention
The present invention relates to a position control method, position control device and medium storage device for controlling the position of an object by suppressing disturbance, and more particularly to a position control method, position control device, and medium storage device for suppressing the position deviation of an object due to disturbance.
2. Description of the Related Art
A position control device is for controlling the position of an object to a specified position and is widely used in various fields. For example, in a medium storage device, such as a magnetic disk device and an optical disk device, it is extremely critical to accurately position the head to a target track to improve recording density.
In this position control, it is known that disturbance affects positioning accuracy. In order to suppress this disturbance by a control system, a method of installing a filter for suppressing specific frequencies (e.g. see U.S. Pat. No. 6,487,028 B1, R. J. Bickel and M. Tomizuka: “Disturbance observer based hybrid impedance control”, (Proceedings of the American Control Conference, 1995, pp. 729-733)), and a method of suppressing such disturbance as eccentricity by observer control (e.g. see Japanese Patent Application Laid-Open No. H7-50075 and Japanese Patent Application Laid-Open No. 2000-21104) have been proposed.
On the other hand, in order for the positioning control system to operate accurately, calibration of gain of open loop characteristics (open loop gain) is indispensable. The open loop gain changes if the characteristics of elements constituting a feedback loop change (e.g. temperature, age-based deterioration). Always maintaining this open loop gain at an optimum contributes to the performance of the feedback loop. This calibration is performed when the power of a device is turned ON, when temperature is changed, or when a predetermined time elapsed, for example, so as to calibrate the open loop gain at an optimum.
As a method for calibrating a gain, applying a sine wave disturbance to a position or current and acquiring and comparing waveforms before and after applying the sine wave disturbance so as to measure the gain of the open loop characteristic is known (e.g. see Japanese Patent Application Laid-Open No. H11-328891, Japanese Patent Application Laid-Open No. H8-167160, and Japanese Patent Application Laid-Open No. H11-96704).
In the case of this conventional gain calibration method, a target gain of the loop characteristic for adjusting the gain and a sine wave disturbance frequency for adjusting the gain are fixed values. In other words, in the case of the conventional gain calibration method, gain is calibrated assuming that the position control system has only one type of characteristic during gain adjustment.
However in the case of a position control system with a disturbance suppression function, the characteristics of the position control system change according to the disturbance frequency to be suppressed. For example, in a control system using adaptive control, the loop characteristic of the control system is different from that before disturbance is applied, when the position control system is following up disturbance based on the adaptive control in a state of applied disturbance vibration.
In order to execute gain adjustment while this adaptive control is following up disturbance, it is necessary to stop the adaptive control and switch to the control system for gain adjustment. In this case, the external vibrations cannot be sufficiently suppressed and positioning accuracy drops since the adaptive control is stopped. As a result, accuracy of gain adjustment drops.
In other words, in the case of such a position control system as an adaptive control system, disturbance is suppressed by changing the loop characteristic according to disturbance, so it is difficult to accurately calibrate the open loop gain of the control system using prior art. Particularly if suppression width is taken wide or if disturbance in a high frequency area is suppressed to meet the recent demand for adapting a wide range of disturbance frequencies, the original characteristics of the controller are influenced, so it is more difficult to calibrate gain accurately.